A crown is a type of dental restoration which completely caps or encircles a tooth or dental implant. Crowns are often needed when a large cavity threatens the ongoing health of a tooth. They are typically bonded to the tooth using a dental cement. Crowns can be made from many materials, which are usually fabricated using indirect methods. Crowns are often used to improve the strength or appearance of teeth. While unarguably beneficial to dental health, the procedure and materials can be relatively expensive.
The most common method of crowning a tooth involves using a dental impression of a prepared tooth by a dentist to fabricate the crown outside of the mouth. The crown can then be inserted at a subsequent dental appointment. Using this indirect method of tooth restoration allows use of strong restorative materials requiring time-consuming fabrication methods requiring intense heat, such as casting metal or firing porcelain which would not be possible to complete inside the mouth. Because of the expansion properties, the relatively similar material costs, and the cosmetic benefit, many patients choose to have their crown fabricated from gold.
As new technology and materials science have evolved, computers are increasingly becoming a part of crown fabrication, such as in CAD/CAM dentistry.
Crowns can also be installed on an implant. A dental implant (also known as an endosseous implant or fixture) is a surgical component that interfaces with the bone of the jaw or skull to support a dental prosthesis such as a crown, bridge, denture, facial prosthesis or to act as an orthodontic anchor. The basis for modern dental implants is a biologic process called osseointegration, in which materials such as titanium form an intimate bond to bone. The implant fixture is first placed so that it is likely to osseointegrate, then a dental prosthetic is added. A variable amount of healing time is required for osseointegration before either the dental prosthetic (a tooth, bridge or denture) is attached to the implant or an abutment is placed which will hold a dental prosthetic.
For individual tooth replacement, an implant abutment is first secured to the implant with an abutment screw. A crown (the dental prosthesis) is then connected to the abutment with dental cement, a small screw, or fused with the abutment as one piece during fabrication. Dental implants, in the same way, can also be used to retain a multiple tooth dental prosthesis either in the form of a fixed bridge or removable dentures.
An implant supported bridge (or fixed denture) is a group of teeth secured to dental implants so the prosthetic cannot be removed by the user. Bridges typically connect to more than one implant and may also connect to teeth as anchor points. Typically the number of teeth will outnumber the anchor points with the teeth that are directly over the implants referred to as abutments and those between abutments referred to as pontics. Implant supported bridges attach to implant abutments in the same way as a single tooth implant replacement. A fixed bridge may replace as few as two teeth (also known as a fixed partial denture) and may extend to replace an entire arch of teeth (also known as a fixed full denture). In both cases, the prosthesis is said to be fixed because it cannot be removed by the denture wearer.
Yttria-stabilized zirconia, also known simply as zirconia is a very hard ceramic that is used as a strong base material in some full ceramic restorations. Zirconia is relatively new in dentistry and the published clinical data is correspondingly limited. The zirconia used in dentistry is zirconium oxide which has been stabilized with the addition of yttrium oxide. Yttria-stabilized zirconia is also known as YSZ.
The zirconia substructure (core) is usually designed on a digital representation of the patient's mouth, which is captured with a 3D digital scan of the patient, impression, or model. The core is then milled from a block of zirconia in a soft pre-sintered state. Once milled, the zirconia is sintered in a furnace where it shrinks by twenty percent and reaches its full strength of 850 MPa (MegaPascals) to 1000 MPa.
The zirconia core structure can be layered with tooth tissue-like feldspathic porcelain to create the final color and shape of the tooth. Because bond strength of layered porcelain fused to zirconia is not strong, “monolithic” zirconia crowns are often made entirely of the zirconia ceramic with no tooth tissue-like porcelain layered on top. Zirconia is the hardest known ceramic in industry and the strongest material used in dentistry. Monolithic zirconia crowns tend to be highly opaque in appearance, while lacking translucency and fluorescence. For the sake of appearance, many dentists will not use monolithic crowns on anterior (front) teeth.
To a large extent, materials selection in dentistry determine the strength and appearance of a crown. Some monolithic zirconia materials produce the strongest crowns in dentistry (the registered strength for some zirconia crown materials is near 1000 MPa.), but these crowns are not usually considered to be natural enough for teeth in the front of the mouth; though not as strong, some of the newer zirconia materials are of better appearance, but they are still not generally as aesthetically acceptable as porcelain fused crowns. When porcelain is fused to the zirconia core, these crowns are more natural than the monolithic zirconia crowns but they are not strong. By contrast, when porcelain is fused to glass infiltrated alumina, crowns are very natural-looking and very strong, though not as strong as monolithic zirconia crowns. Another monolithic material, lithium-disilicate, produces extremely translucent leucite-reinforced crowns that often appear to be too gray in the mouth, and to overcome this, the light shade polyvalent colorants are added, which take on a distinctly unnatural, bright white appearance. Other crown material properties to be considered are thermal conductivity and radiolucency. Stability/looseness of fit on the prepared tooth and cement gap at the margin are sometimes related to materials selection, though these crown properties are also commonly related to system and fabricating procedures.
Given that each patient recipient of a crown has teeth that are uniquely colored from a lifetime of stains, a zirconia ingot of the proper color must be matched to the patient's remaining teeth. As a general rule, materials used to create crowns are available in ten different shades. This is also true of the puck-shaped zirconia ingots from which the crowns are milled. They are typically provided in packages, which do not lend themselves to ease of access. What is needed for dental laboratories is a wall-mounted holder/dispenser for zirconia ingots which provides ready access to each of the ten color shades of ingots.